In the field of ribbed pneumatic tires for high-speed driving in heavy-load vehicles, trials have been carried out to improve braking capacity on a wet road, that is, the wet braking capacity, without reduction of the abrasion resistance by making complex tread patterns.
These trials are roughly divided into the following two means.
(1) Many continuous zigzag grooves extending in the circumferential direction of a tire are formed on the tread surface of the tire while leaving spaces in the lateral direction.
(2) The point height Hm of the many zigzag grooves formed on the tread surface of a tire is increased.
However, these means involve problems as described below.
(a) In the above-mentioned means (1), the braking effect depends on the number and width of the zigzag grooves arranged on the tread surface of the tire. Accordingly, if many grooves having a broad width are arranged in a limited ground-contacting region to improve the braking effect, the actual ground-contacting area of the tire T is decreased and the abrasion resistance is reduced. Furthermore, if the groove width is increased, abnormal abrasion, known as "rib punching", occurs and the abrasion resistance of the tire is drastically reduced.
(b) The above-mentioned means (2) is advantageous over the means (1) in that the braking effect can be improved without increasing the number and width of the grooves. However, since the width of ribs Rs formed on the side of the shoulder portion is greatly changed with the increase of the point height Hm of the grooves, abnormal abrasion, known as "wavy abrasion", occurs, also, in the means (2), the abrasion resistance of the tire is drastically reduced. Moreover, the riding comfort and driving stability are reduced.